September 15, 2009 / Vol. 34, No. 18 / OPTICS LETTERS
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Mode-locked Yb-doped Bragg fiber laser Caroline Lecaplain,1 Ammar Hideur,1,* Sébastien Février,2 and Philippe Roy2 1
CNRS UMR 6614 CORIA, Avenue de l’Université, BP 12, 76801 Saint Etienne du Rouvray, France 2 Xlim, UMR 6172 CNRS, Université de Limoges, 87060 Limoges, France *Corresponding author:
[email protected]
Received May 27, 2009; accepted July 23, 2009; posted August 20, 2009 (Doc. ID 112007); published September 15, 2009 We report on a mode-locked fiber laser featuring an Yb-doped large-mode-area Bragg fiber and exploiting dissipative-soliton pulse shaping. Reliable self-starting mode-locking is achieved using a fast semiconductor saturable absorber mirror. The laser generates 30 nJ chirped pulses at 17 MHz repetition rate for an average power of 510 mW. The 3.2 ps output pulses are compressed outside the cavity to 440 fs. © 2009 Optical Society of America OCIS codes: 320.7090, 320.5540, 140.7090, 060.2400.
Femtosecond fiber lasers are attractive short-pulse sources because of their stability, low sensitivity to alignment, compact design, and low production cost. In recent years, femtosecond fiber lasers have been developed with ever increasing performance in terms of pulse duration and energy. In particular, fiber lasers operating in the 1 m range have experienced significant progress owing to the exceptional efficiency and the gain bandwidth of ytterbium-doped fibers. However, because of the tight confinement of light over long lengths, nonlinear effects, mainly Kerr nonlinearity, hinder self-consistent pulse evolution inside a fiber oscillator at high energies. Various strategies have been developed to overcome this limitation, thereby allowing for an increase of pulse energy from femtosecond fiber laser oscillators. The traditional approach draws on the use of laser cavities operating in normal path-averaged group velocity dispersion (GVD). This tends to scale down the peak power inside the fiber core by stretching the pulse during its propagation, so that nonlinear phenomena can be kept under control. This concept is implemented in operation regimes such as the stretched-pulse regime [1] and the self-similar regime [2], where the pulse width experiences large variations per cavity round trip. Similariton fiber lasers could generate sub-100 fs pulses with over 10 nJ of energy [2,3]. The highest pulse energies are obtained from purely normal dispersion lasers [4,5]. Such lasers support dissipative solitons and could produce energies as high as 30 nJ using standard fibers [6,7]. More recently, exceptional performance in terms of pulse energy and peak power have been demonstrated in mode-locked lasers based on largemode-area (LMA) microstructure fibers [8–12]. Pulse energies approaching the microjoule level have been demonstrated in a rod-type microstructure fiber laser [12]. However, LMA microstructure fibers are obtained at the cost of weakened waveguiding, leading to strong bend sensitivity. Recently, a new LMA fiber design based on the discrimination of transverse modes in a photonic bandgap fiber has been demonstrated [13]. This Bragg fiber consists of a low-index core surrounded by a periodic cladding composed of alternating low-n and high-n layers. To engineer the fiber, there exist many degrees of freedom such as 0146-9592/09/182879-3/$15.00
core diameter, lattice constant, thickness and index contrast of high-n layers, and number of bilayers. For a fixed core diameter, appropriate values of these parameters can be found making the photonic bandgap fiber single mode. Moreover, because of a strong confinement of the fundamental mode, Bragg fibers have weak bend sensitivity despite large bending radii [13]. Février et al. have recently achieved 7 W average power from an Yb-doped Bragg fiber laser with good beam quality and very low bend sensitivity [14]. In this Letter, we report the first achievement of femtosecond pulses from a mode-locked Bragg fiber laser. In the solitonlike regime, femtosecond pulses with 610 fs duration and 14 nJ energy are directly generated at the laser output. In the all-normal dispersion regime, the laser delivers an average output power of 510 mW at a repetition rate of 17 MHz, which corresponds to 30 nJ energy per pulse. Positively chirped pulses with 3.2 ps duration are generated. After dechirping outside the cavity, 440 fs pulses are obtained. The experimental setup of the laser is shown in Fig. 1. The Bragg fiber was fabricated at the Institute of High Purity Substances and Fiber Optics Research Center, Russian Academy of Sciences. [14]. It consists of an Yb-doped core of 20 m diameter surrounded by a periodic cladding composed of alternating low-n and high-n layers. The three high-n layers are GeO2-doped. Adjusting lattice constant ⌳, thickness d, and index contrast ⌬n of high-n layers, confinement loss of the fundamental mode is made low 共 ⬃ 1 dB/ m兲, whereas high-order mode losses are
Fig. 1. (Color online) Schematic of the passively modelocked fiber laser: HWP, half-wave plate; QWP, quarterwave plate; DM, dielectric mirrors; SAM, saturable absorber mirror. Insets, (a) cross-section of the Yb-doped Bragg fiber and (b) output beam profile. © 2009 Optical Society of America
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made higher than 10 dB/ m. The effective area of the fundamental mode is close to 200 m2. The inner cladding diameter is 120 m. The 1.2-m-long gain fiber is cladding-pumped with a fiber-coupled laser diode emitting at 976 nm. The laser cavity is mounted in a sigma configuration using a polarizationsensitive optical isolator (PS-ISO). A dichroic mirror M1 is used to separate the pump beam from the laser beam. The gain fiber is located at the ring portion of the cavity. Passive mode locking is achieved by using a high-nonlinearity fast saturable absorber (SAM) introduced in the sigma branch. The SAM is based on an InGaAs multi-quantum-well structure grown on a multilayer GaAs/ AlAs Bragg mirror by the lowtemperature molecular beam epitaxy method. The low-intensity absorption of the SAM around 1040 nm is 65%, the modulation depth is 35%, and the saturation fluence is 20 J / cm2. The relaxation time of the SAM is shorter than 500 fs. The antiresonant design of the SAM ensures a broad absorption bandwidth of more than 40 nm FWHM. Mode locking is obtained by optimizing the saturation criteria on the saturable absorber using an adequate focusing lens. The rejected port of the isolator serves as a variable output leading to linearly polarized output power. A polarization controller including a half- and a quarter-wave plate is used to adjust the output coupling coefficient by optimizing the output power performances. A mirror array is used to increase the length of the linear part of the cavity, resulting in a pulse repetition rate of 17 MHz. We first studied the operation of the laser in the solitonlike regime obtained for an anomalous overall dispersion of the cavity. To do so, we inserted a grating pair inside the cavity located in the sigma branch. Adjusting the grating separation distance controls the averaged-cavity dispersion. The results obtained for a grating dispersion of approximately −0.145 ps2 are shown in Fig. 2. The optical spectrum displays soliton sidebands that are characteristic of the solitonlike regime. The pulse duration is 610 fs assuming a sech2 pulse profile. The highest output power measured in the single-pulse operation is 250 mW (14.5 nJ pulse energy), obtained for an output coupling ratio of ⬃40%. The amplitude noise level measured using an rf spectrum analyzer is lower than 0.55%. We have further estimated the dispersion of the Bragg fiber using the sideband method. By repeating the measurement for various gratings separation, we estimated an average value for the Bragg fiber dispersion of +0.06 ps2 / m. This value is
Fig. 2. Typical optical spectrum obtained in (a) the solitonlike regime and (b) corresponding autocorrelation trace.
about two times higher than the GVD in standard fibers. It has been demonstrated that mode coupling between the fundamental core mode and some odd ring modes could lead to large chromatic dispersion, provided antisymmetric perturbation such as curvature is applied [15]. In our case, the high dispersion value was attributed to the large bending diameter 共 ⬃ 30 cm兲. It is worth noting, however, that the short fiber length used 共1.2 m兲 prevents a strong coupling from occuring, thereby ensuring a good beam quality as shown in the inset of Fig. 1. To explore the energy scaling potential of our fiber, we have constructed an all-normal dispersion cavity as shown in Fig. 1. First experimental studies dealt with the optimization of the output coupling ratio and the focusing conditions of the SAM. Optimal operation is obtained for an output coupling of ⬃60%. Under these conditions, the self-starting modelocking regime is reached for an average output power of ⬃120 mW. The laser delivers a single-pulse train at a repetition rate of 17 MHz. The average power could be increased up to an output average power of 510 mW (30 nJ energy) for a launched pump power of ⬃5 W. A further power increase results in Q-switching instabilities. We note that the threshold of these instabilities remains unchanged when increasing the focal spot size on the SAM, indicating that this behavior did not result from the SAM overdrive. The lasing threshold of the cavity without feedback corresponds to the appearance of the Q-switching instabilities, indicating that the latter could be due to backreflection from the fiber ends. So, higher energies are expected in the single-pulse operation by enhancement of the fiber end preparation. Figure 3(a) shows the typical optical spectrum obtained for an average output power of 500 mW. It is centered at 1035 nm wavelength with a spectral width (FWHM) of 6.5 nm. The optical spectrum ex-
Fig. 3. (Color online) Output of the all-normal dispersion laser (solid curves): (a) main output spectrum, (b) output pulse autocorrelation on linear and logarithmic (insets) scales,(c) autocorrelation of the dechirped pulse, (d) pulse dynamics inside the cavity from numerical simulation. Also shown are the results from numerical simulations (open circles). OC, output coupler. SA, saturable absorber.
September 15, 2009 / Vol. 34, No. 18 / OPTICS LETTERS
hibits a multipeaked shape with steep edges, typical for all-normal dispersion fiber lasers. The typical output pulse autocorrelation is shown in Fig. 3(b). The high contrast of the autocorrelation trace [see inset of Fig. 3(b)] confirms the good quality of laser emission, which takes place mainly on the fundamental core mode. The corresponding pulse duration is 3.2 ps assuming a sech2 pulse shape. The output pulses are then dechirped outside the cavity using bulk gratings. The dechirped pulse duration is 440 fs [see Fig. 3(c)]. The corresponding time–bandwidth product is 0.8. The average output power of the dechirped pulses is 350 mW, which corresponds to a peak power of more than 46 kW. The amplitude noise level is lower than 0.45%, which confirms the stable modelocking operation. To investigate the pulse evolution in the cavity, we performed numerical simulations using the standard split-step method with the arrangement of the laser cavity elements shown in Fig. 3(d). The absorption in the semiconductor was described by the rate equation model considering a decay time of 500 fs [16]. The gain bandwidth of the Yb-doped Bragg fiber was approximated by the FWHM of the laser’s tuning bandwidth measured in the cw regime, which is ⬃20 nm. The numerical results show that stable pulse solutions do exist for output pulse energies varying from a few nanojoules to more than 100 nJ. In particular, results of a numerical simulation performed with accurate fiber parameters and 30 nJ pulse energy are shown in Fig. 3(d). The intracavity pulse evolution shows clearly that pulse shaping is dominated by the SAM nonlinearity. The pulse duration increases almost monotonically in the gain fiber, and the pulse shortening ocurring in the saturable absorber ensures its self-consistency over one cavity round-trip. The most significant insight is that spectral self-consistency results from mutual actions of the SAM nonlinearity and the gain filtering effect. The pulse breathing ratio is close to unity, which is typical for dissipative solitons in normal dispersion fiber lasers. The spectrum exhibits an asymmetric shape with sharp edges [see Fig. 3(a)]. However, the lateral side-peaks observed on the power spectrum are not predicted numerically. The structured spectral shape could be attributed to nonlinear polarization evolution (NPE) effects [17]. Further numerical and experimental work is now needed to study the impact of the polarization effects on laser performance. Nevertheless, the numerical results present good qualitative agreement with experimental ones. Indeed, temporal profiles as well as pulse durations calculated before and after compression are very close to those of experiments (Fig. 3). Interestingly, numerical simulations show that pulse energies of more than 100 nJ could be achieved in this fiber laser. In conclusion, we demonstrated a self-starting mode-locked laser featuring a large-mode-area Yb-
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doped Bragg fiber. Pulse energy as high as 30 nJ at a repetition rate of 17 MHz was obtained. The laser operates in the dissipative soliton regime and generates 3.2 ps pulses, which are dechirped outside the cavity down to 440 fs. We believe that Bragg fibers are very promising for greatly extending the energy extracted from mode-locked fiber lasers while maintaining the primary benefits of the fiber medium. This work is supported by the French Agency for Research (ANR) under projects OFFEMET (ANR-06JCJC-116) and HiPoLYFF (ANR-06 BLAN-0091-01). We acknowledge support from the Conseil Régional de Haute Normandie and the GDR-PhoNoMi2 Network. The authors warmly thank M. E. Likhachev and E. M. Dianov from Fiber Optics Research Center, Russian Academy of Sciences, for the fabrication of the fiber. References 1. K. Tamura, L. E. Nelson, H. A. Haus, and E. P. Ippen, Opt. Lett. 18, 1080 (1993). 2. F. Ö. Ilday, J. R. Buckley, F. W. Wise, and W. G. Clark, Phys. Rev. Lett. 92, 213902 (2004). 3. J. R. Buckley, F. W. Wise, F. Ö. Ilday, and T. Sosnowski, Opt. Lett. 30, 1888 (2005). 4. L. M. Zhao, D. Y. Tang, and J. Wu, Opt. Lett. 31, 1788 (2006). 5. A. Chong, J. Buckley, W. Renninger, and F. Wise, Opt. Express 14, 10095 (2006). 6. J. An, D. Kim, J. W. Dawson, M. J. Messerly, and C. P. J. Barty, Opt. Lett. 32, 2010 (2007). 7. K. Kieu, W. H. Renninger, A. Chong, and F. W. Wise, Opt. Lett. 34, 593 (2009). 8. B. Ortaç, J. Limpert, and A. Tünnermann, Opt. Lett. 32, 2149 (2007). 9. C. Lecaplain, C. Chédot, A. Hideur, B. Ortaç, and J. Limpert, Opt. Lett. 32, 2738 (2007). 10. B. Ortaç, O. Schmidt, T. Schreiber, J. Limpert, A. Tünnermann, and A. Hideur, Opt. Express 15, 10725 (2007). 11. Y.-J. Song, M.-L. Hu, C.-L. Wang, Z. Tian, Q. R. Xing, L. Chai, and C.-Y. Wang, IEEE Photon. Technol. Lett. 20, 1088 (2008). 12. B. Ortaç, M. Baumgartl, J. Limpert, and A. Tünnermann, Opt. Lett. 34, 1585 (2009). 13. S. Février, R. Jamier, J.-M. Blondy, S. L. Semjonov, M. E. Likhachev, M. M. Bubnov, E. M. Dianov, V. F. Khopin, M. Y. Salganskii, and A. N. Guryanov, Opt. Express 14, 562 (2006). 14. S. Février, Dmitry D. Gaponov, P. Roy, M. E. Likhachev, S. L. Semjonov, M. M. Bubnov, E. M. Dianov, M. Yu. Yashkov, V. F. Khopin, M. Yu. Salganskii, and A. N. Guryanov, Opt. Lett. 33, 989 (2008). 15. F. Gérôme, S. Février, A. D. Pryamikov, J.-L. Auguste, R. Jamier, J.-M. Blondy, M. E. Likhachev, M. M. Bubnov, S. L. Semjonov, and E. M. Dianov, Opt. Lett. 32, 1208 (2007). 16. N. N. Akhmediev, A. Ankiewicz, M. J. Lederer, and B. Luther-Davies, Opt. Lett. 23, 280 (1998). 17. C. Chédot, C. Lecaplain, S. Idlahcen, G. Martel, and A. Hideur, Fiber Integr. Opt. 27, 341 (2008).
